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Half the universe is missing

TODAY’S universe is only half the place it used to be. Admittedly there is still a lot of it around, locked up in dust and gas, but that’s only half the story – quite literally. Over 13 billion years ago, the big bang forged equal amounts of matter and antimatter. Take a look around today though, and you’ll see that all the antimatter has vanished.

It’s lucky for us that it has. According to our best theories, all the matter and antimatter should have annihilated each other in a puff of radiation. The universe should be filled with pure light, not planets and stars and people. To create the universe we see today, matter somehow gained the upper hand over antimatter a fraction of a second after the big bang. But how?

Five shiny germanium cylinders sitting in a cavern under a mountain in central Italy hold the answer, or at least the physicists in charge of them believe they do. Hans Klapdor-Kleingrothaus and his colleagues from the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have been monitoring the cylinders day and night for over a decade in the hope of spotting a form of radioactive decay that, according to all received wisdom, should not exist. And earlier this year they claimed to have made nearly 30 sightings of it. “If confirmed, this discovery could be worth a Nobel prize,” says Petr Vogel, a nuclear physicist at the California Institute of Technology in Pasadena.

What the Heidelberg group claim to have seen is a peculiar form of beta decay. In normal beta decay, a neutron inside an atomic nucleus …